Ocarina of Time because it's soundtrack had the most memorable themes in the Zelda series (Lost woods, Gerudo Valley, Zora's Domain etc). Every track is amazing, distinct and goes well with the location/dungeon.
I don't know why, but I just love the overall feel of Spirit Track's soundtrack, especially the main theme and final battle music. I think I'd choose Skyward Sword for second place, since I really love orchestrated video game music. The original Zelda and OoT both have the most memorable soundtracks, though... GAH, IT'S SO HARD TO CHOOSE!